Support for a base url option

[bmarsaud]

Hi! Thanks for your work on this image.

I'm publishing an SPA on a subpath, let's say on https://example.com/app/. I want to serve :

• My index.html on /app
• My static assets on /app/assets/xxxx
• Redirect my SPA routes (/app/route1 for instance) to the index.html

For now, my static assets are not served properly because it will search for /app/assets/xxxx files ont the fs instead of /assets/xxxx.
A base url option, would allow to match the incoming URL with the file path.

A workaround is to copy the static assets to the "base url path" in the image fs while keeping the index.html on root :

FROM devforth/spa-to-http:latest AS runner
ARG BASE_URL="app"
RUN mkdir -p ${BASE_URL}
COPY --from=build /usr/src/app/dist ${BASE_URL}
RUN mv ${BASE_URL}/index.html index.html

But I think a runtime option would make it cleaner.
It's the same issue as #11 that was closed

Would like to submit a PR but i don't know GO tbh!